The present invention relates to a cathode capable of electrolyzing aqueous alkali metal halide and alkali metal hydroxide solutions at a low hydrogen overvoltage for a long period of time, and also pertains to a method of producing the low-hydrogen overvoltage cathode.
When an aqueous alkali metallic halide solution or an aqueous alkali metal hydroxide solution is electrolyzed by the diaphragm process, the ion-exchange membrane process, or the like, hydrogen is evolved at the cathode. Hitherto, the cathode has been formed using a material composed mainly of soft steel or stainless steel. However, the cathode made of soft steel or stainless steel has the drawback that the hydrogen overvoltage is high. For this reason, various types of low-hydrogen overvoltage cathodes have been proposed.
The low-hydrogen overvoltage cathode is demanded to be capable of stably sustaining a low hydrogen overvoltage for a long period of time, to cause no elution of impurities in the catholyte at the time of starting an electrolytic cell operation or during the cell operation, not to ignite or deteriorate its performance characteristics on contact with the air during dismantling of the electrolytic cell or under other circumstances, and to be less costly.
A cathode coated with Raney nickel or Raney nickel and a hydrogen-occluding alloy by composite plating, as disclosed in Japanese Patent Application Post-Examination Publication No. 61-12032 (1986) or 61-36590 (1986), has an advantage in that the hydrogen overvoltage is low. However, the disclosed cathode suffers from the problem that it may ignite or deteriorate its performance characteristics on contact with the air during dismantling of the electrolytic cell or under other circumstances. Japanese Patent Application Post-Examination Publication No. 63-4920 (1988) and Japanese Patent Application Laid-Open (KOKAI) No. 62-253791 (1987) disclose cathodes which are electroplated with a nickel-tin alloy, and Japanese Patent Application Laid-Open (KOKAI) No. 62-284094 (1987) discloses a cathode which is electroplated with a nickel-chromium alloy. These cathodes suffer, however, from the problems that the hydrogen overvoltage is higher than that of the cathode that is composite-plated with Raney nickel or Raney nickel and a hydrogen-occluding alloy, and that tin and chromium ions are eluted in the catholyte at the time of starting an electrolytic cell operation or during the cell operation, and further that the cathode deteriorates its performance characteristics on contact with the air.
Japanese Patent Application Post-Examination Publication Nos. 25-2305 (1950) and 02-60759 (1990), Japanese Patent Application Laid-Open (KOKAI) No. 62-93389 (1987), etc. disclose cathodes which are formed with a nickel layer containing sulfur by carrying out electroplating in a nickel plating bath containing a sulfur compound. Japanese Patent Application Laid-Open (KOKAI) No. 57-35689 (1982) discloses a cathode formed with a nickel layer by carrying out electroplating in a nickel plating bath containing a sulfur compound and having active carbon particles dispersed therein. These cathodes have an advantage in that the performance characteristics are not deteriorated even when the cathodes come in contact with the air, but they suffer from the problems that the hydrogen overvoltage is higher than that of the active cathode which is composite-plated with Raney nickel or Raney nickel and a hydrogen-occluding alloy, and that sulfur ions are eluted in the catholyte at the time of starting an electrolytic cell operation or during the cell operation.
Japanese Patent Application Laid-Open (KOKAI) No. 57-89491 (1982) discloses a method wherein an electrode active layer is formed by carrying out nickel plating in a nickel plating bath containing platinum metal ions and having active carbon particles dispersed therein. With this method, however, since the platinum metal coprecipitates with nickel, the surface area of the platinum metal, which comes in contact with the catholyte, cannot be increased. Therefore, it is impossible to make full use of the characteristics of the platinum metal.
Japanese Patent Application Post-Examination Publication Nos. 63-64518 (1988), 03-35387 (1991), etc. disclose cathodes which are coated with a nickel oxide or a cobalt oxide by flame spraying. These cathodes have the advantageous features that no impurities are eluted in the catholyte at the time of starting an electrolytic cell operation or during the cell operation, and that the performance characteristics of the cathode are not deteriorated even when it comes in contact with the air. However, the disclosed cathodes have the problem that the hydrogen overvoltage is higher than that of the cathode that is composite-plated with Raney nickel or Raney nickel and a hydrogen-occluding alloy.
Further, various types of low-hydrogen overvoltage cathode formed with an electrode active layer containing a platinum metal have been proposed. For example, Japanese Patent Application Post-Examination Publication Nos. 63-41994 (1988) and 61-45711 (1986) disclose low-hydrogen overvoltage cathodes which use a platinum metal in the form of a Raney metal type alloy and, hence, have an advantage in that the cathode activity is high even if the amount of platinum metal used is relatively small. However, these cathodes suffer from the problem that when they come in contact with the air, the cathodes may ignite, or the cathode activity may deteriorate. Japanese Patent Application Post-Examination Publication Nos. 61-36591 (1986) and 61-48582 (1986) disclose low-hydrogen overvoltage cathodes having a cathode active layer formed of a platinum metal by electroplating. However, since the cathode active layer is formed in a two-dimensional manner, the cathode activity is disadvantageously low.
It is an object of the present invention to provide a cathode having a low-hydrogen overvoltage, capable of stably sustaining the low-hydrogen overvoltage for a long period of time, having no possibility of its performance characteristics being deteriorated even when it is taken out of an electrolytic cell, and causing no elution of a substance contaminating the electrolyte at the time of starting an electrolytic cell operation or during the cell operation.